This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The flight muscle of the Hawkmoth Manduca sexta is interesting for two reasons 1) It's active force/extension curve in very similar to mammalian cardiac muscle 2) there is an astonishing temperature gradient in the dorso-ventral direction, with a mean difference of 8.8 [unreadable]C (a max of 10 [unreadable]C;n = 7) across 5 mm. It is known that during muscle contraction, heat is produced as chemical energy is converted into mechanical work. Many large insects with active flight muscles use this byproduct to elevate flight muscle temperature, thereby achieving higher mechanical power output during flight. Contractile heat production paired with convective and radiative heat loss necessarily lead to a temperature gradient, but the functional consequences of such a gradient remain unknown. Because force generation of muscle depends on temperature, subunits experiencing lower temperatures could function differently than those at higher temperatures. Do thermal gradients necessarily imply fundamental structural differences between fibers in different parts of muscle? Test the hypothesis that crossbridges in cold muscle do not detach rapidly as rapidly as in hot muscle. i.e. detachment rate a temperature dependent phenomenon. The attachment/detachment rates of crossbridges will also depend on elastic storage in a given muscle.